The invention is based on the use of the known laser-Doppler technique for measuring the superficial circulation of blood in cutaneous tissue. This technique is described, for instance, in U.S. Pat. Nos. 3,511,227, 4,109,647, SE 419 678 and tile articles "In Vivo Evaluation of Microcirculation by Coherent Light Scattering", Stern, N. D., Nature, Vol. 254, pp. 56-58, 1975; "A New Instrument for Continuous Measurement of Tissue Blood Flow by Light Beating Spectroscopy", Nilsson, G. E., Tenland, T. and Oberg, P. A., IEE trans., BME-27, pp. 12-19, 1980, and "Evaluation of a Laser Doppler Flow Meter for Measurement of Tissue Blood Flow", Nilsson, G. E., Tenland, T. and Oberg, P. A., IEE trans., BME-27, pp. 597-604, 1980. In principle, this technique involves directing a laser beam onto a part of the tissue and receiving, with the aid of an appropriate photodetector, part of the light scattered and reflected back by that part of the tissue that is irradiated by the laser beam. As a result of the Doppler effect, the frequency of the reflected and scattered light is broadened and the frequency spectrum of the light will thus be broader than the frequency spectrum of the original laser beam, this broadening of the light frequency being due to the influence of movement of blood cells in the superficial part of the irradiated tissue. The extent to which the frequency is broadened and the intensity of light within different parts of this broader frequency spectrum constitute a measurement of the superficial blood circulation in the irradiated part of the examined tissue and can be determined or evaluated by appropriate processing of the photodetector output signal.
Swedish Patent Application No. 8903641-2 describes how this technique is used in a system for measuring and visually presenting the extent of superficial blood circulation over a large area of a part of the body, for instance a complete hand or foot or a part of a hand or foot or a part of a leg. The superficial blood circulation can vary quite considerably within different regions of a body part, and the described system enables the course taken by an illness, disease or healing process to be studied effectively. This known system includes a laser light source for generating a laser beam and means for directing the laser beam onto the body part to be examined and for guided movement of the laser beam over said body part in accordance with a predetermined scanning pattern. The system also includes means for receiving light reflected from said body part and for detecting the broadening in frequency of the reflected light caused by the Doppler effect, and also for registering this broadening of the frequency over a large number of points along the path scanned by the laser as a measurement of the superficial blood circulation in said body part at said points. The system also includes means for visual presentation on a color screen of the magnitude of the superficial blood circulation at the scanned points, using mutually different colors for mutually different blood circulation magnitude intervals.
When practicing the light-fiber-based Doppler effect described in the introduction, registration of the superficial blood circulation is effected solely with the aid of a punctiform measuring process. In the technique described in Swedish Patent Application No. 8903641-2, the laser beam scans different measuring points over a wide surface area, so as finally to generate an image or picture of the microcirculation of the surface scanned. In order to be able to compare mutually all of the measurement values registered at the various measurement points over this area, it is necessary for the different conditions that prevail during the measuring procedure to be equal when taking the comparison measurements. This is often not the case with regard to the position of the detector in relation to the measurement points on the measured object, which leads to uncertainty when making a comparison study between the values measured on different occasions. In the case of the image-producing system in which the laser beam scans the measurement object, the distance between the measurement object point and the detector surface, and therewith also the angle of the reflected beam in relation to the detector, will vary during scanning of the body part. Consequently, the system amplification factor for the measured signal will vary within one and the same image, or picture, which introduces a distortion in the reproduction of the blood flow image.
The theory of how such distance-dependent-amplification occurs is described comprehensively in Swedish Patent Application No. 9002467-0. It is described in this application that, in addition to the angle .alpha. between the normal of the detector surface and the line from which the light spot on the object is seen from the detector, the distance-dependent-amplification is mainly due to the fact that the size of the coherence area (A.sub.coh) on the detector surface is a function of the solid angle (.OMEGA.) at which the light spot on the object is seen from the detector, in accordance with the formula: EQU A.sub.coh =.lambda..sup.2 /.OMEGA. (1)
The solid angle .OMEGA. a is dependent on the distance X between the detector and the measurement object, the diameter of the light spot on the object (d.sub.s) and the wave length .lambda. according to the equation EQU A.sub.coh =(4.lambda.X/d.sub.s).sup.2 ( 2)
Described in Swedish Patent Application No. 9002467-0 is a method for compensating this distance-dependent coherence-area size and therewith system amplification. In this application, the number of coherence areas N and therewith the amplification is described as a function of the distance X between measurement object and detector, in accordance with the equation EQU N=.pi.(r.sub.d r.sub.s /2.lambda.X).sup.2 cos .alpha. (3)
where
r.sub.d =detector radius, PA1 r.sub.s =laser beam radius.
When the beam is located immediately beneath the detector, the number of coherence areas will be EQU N=.pi.(r.sub.d r.sub.s /2.lambda.D).sup.2
where D is the perpendicular distance between detector plane and measurement object.
According to the application, this distance D can be measured by registering the time taken for an ultrasonic pulse to travel to the measurement object from an ultrasonic detector placed on a level with the light detector, and reflected back to the ultrasonic detector. The distance X can be calculated on the basis of this time lapse and the atmospheric speed of sound. Since D is constant and known, the size A.sub.coh of the coherence area can also be calculated. When the angle .alpha. between the normal of the detector surface and the direction from which the light spot is seen on the object from the photodetector, it is possible to calculate and compensate for the total number of coherence areas on the detected surface, and therewith also the amplification factor. When carrying out the method in practice, the distance X is calculated for each measurement point, by measuring the perpendicular distance between the detector surface and the measurement object prior to starting collection of the images, and the distance between the measurement object point immediately beneath the detector and the measuring point concerned is calculated, for instance, by detecting the positions of the scanner mirrors.
This method has been found to produce good results with measurement objects which are relatively flat. However, when the scanned surface is irregular, such as the upper skin of a hand, the value of the distance X obtained with this method will deviate from the true value, and there is also a risk that the ultrasonic pulse will be reflected away and not registered by the ultrasonic receiver. Thus, the measuring accuracy of the construction described in Swedish Application No. 9002467-0 is limited in those cases when the measurement object has an irregular or uneven measurement surface.
The Basic Concept of the Invention
The object of the present invention is to solve the aforesaid problems with the aid of a method and a system which are devised so that the amplification factor, and therewith the number of coherence areas on the detector surface, will be kept constant and independent of the distance between detector and measurement object and also independent of the angle .alpha. between the normal of the detector surface and the line along which the light spot is seen from the detector. This will result in correct presentation of the measured flow values, even when the surfaces of the measurement object are rough and irregular. According to the present invention, this is achieved with a method and a system set forth in the following claims.
Variations in the angle .alpha. can be readily compensated for, by measuring the angle continuously and registering the measurements obtained with a starting point from the position of those drive means which control the scanner mirrors, and therewith the position of the light spot. In this case, the angle compensation factor is proportional to cos .alpha..
The invention is based on the concept of a method and a system which will render the size of the coherence area A.sub.coh independent of the distance between detector and measurement object. According to the present invention, this is achieved by maintaining the number of coherence areas on the detecting detector surface constant. According to one preferred embodiment of the invention, this can be achieved by causing the laser beams to diverge so that the diameter of the light spot on the measurement object will be proportional to the distance X between the detector and the measurement object at each measurement point.